Question

How does water in the granular backfill in the wall change /affect the design of an MSE wall?

a) Unit weight of granular fill

b) Total overturning/sliding forces

c) Corrosion rate of reinforcement

d) What would be your choice of soil behind the wall to minimize water-related problems.

Answer 1

* a)AFFECT OF WATER IN THE GRANULAR BACK FILL RELATED TO UNIT WEIGHT OF SOIL.  

TO ACHIEVE BETTER COMPACTNESS, SOIL FILL NEED TO BE COMPACTED TO 95% OF MAXIMUM DRY DENSITY WHICH INDUCES BETTER FRICTION BETWEEN THE SOIL PARTICLES. IF WATER INCRESES IN THE SOIL THE DENSITY OF THE SOIL GETS REDUCED AS THE VOIDS ARE OCCUPIED BY THE WATER. WHEN SURCHAGE LOAD IS APPLIED IN PRESENCE OF MORE WATER PORE PRESSURE DEVELOPS WHICH SHOULD ALSO BE CONSIDERED IN THE DESIGN. IF SOIL IS HIGHLY SATURATED THE FRICTION BETWEEN THE SOIL IS REDUCED WHICH INTURN REDUCES THE STRENGTH OF THE BACKFILL MATERIAL.

* b) OVERTURNING/ SLIDING FORCES

OUTWARD SLIDING USUALLY OCCURS DUE TO THRUST OF UNREINFORCED BACK FILL. THIS OCCURS JUST ABOVE OR BELOW THE BOTTOM LEVEL OF REINFORCEMENT MATERIAL AS THE FRICTION BETWEEN THE SOIL AND REINFORCEMENT MATERIAL IS LOW. IF WATER IS PRESENT IN THE FILL IT AFFECTS THE OVERTURNING AND SLINDING FORCES. USUALLY FOR BOTH SLIDING AND OVERTURNING FACTOR OF SAFTEY USED IS 2. SO WITH PRESENCE OF WATER AS DENSITY REDUCES THE FACTOR OF SAFETY INCREASES.

*c) CORROSION RATE OF REINFORCEMENT

DURING THE DESIGN OF MSE CORROSION ALLOWANCE IS CALCULATED WITH CONSIDERATION OF CORROSION OF THE REINFORCEMENT MATERIAL. WHICH IS BASED ON THE AASHTO CORROSION RATES.

  GALVANIZATION LOSS IS 15micrometer for first 2 years and increases subsiquantly

CARBON STEEL LOSS IS 12 MICRO METRE PER YEAR AFTER ZINC DEPLETION.

CORROSION RATE ACCELERATES IF MOISTURE CONTENT IS MORE IN THE BACK FILL AND IF THE WATER IS SALTY THE CHEMICAL REACTION CAUSING CORROSION WILL BE FASTER.

FOR DETERMINING THE LIFE SPAN OF THE MSE, ITS BETTER TO CONSIDER LOWEST TESTED TENSILE STRENGTH AND HIGHEST LOSS OF CROSS SECTION.

*d) CHOICE OF THE SOIL   

ITS BETTER TO PREFER GRANULAR SOIL BECAUSE OF THE FRICTION BETWEEN THE SOIL PARTICLE WHICH HAS HIGH RESISTANCE TO FAILURE. IN THIS CASE BECUASE OF ITS PERMEABILITY WATER EASILY DRAINS OUT.

IF FINEGRAINED SOIL LIKE CLAY IS USED THAN IT WILL HAVE MUCH PROBLEM. MAIN PROBLEM IS THE SWELL CHARACTERISTIC OF THE SOIL WHICH WOULD CHANGE WITH CHANGE IN MOISTURE CONTENT. IT SWELLS WITH INCREASE IN MOISTURE CONTENT AND SHRINKS WITH DECRESE IN MOISTURE CONTENT. IT DONOT HAVE MUCH FRICTION BETWEEN THE SOIL PARTICLES AND WITH INCREASE IN WATER FRICTION IS FURTHER REDUCED AND IT RESULTS SLIDING FAILURE OF THE BACK FILL MATERIAL.